The transmission quality of a wireless communication and/or coherence properties of a communication channel between two nodes, such as e.g. a base station and a user equipment within a wireless communication system may differ, depending on a plurality of unwanted influence on the signal and the radio propagation conditions. Some non limiting examples of such unwanted influence may be e.g. thermal noise and interference and some examples of phenomena that adversely affect the propagation conditions are path loss, signal multi-path, and Doppler spread. Further the accuracy of channel estimation will affect the transmission quality. Thus data units such as e.g. a Protocol Data Unit (PDU) sent from one node may arrive at the receiving node distorted or not at all. In practice RLC PDUs may be received out of order due to that different RLC PDUs are experiencing different number of Hybrid automatic repeat-request (HARQ) retransmissions which may cause reordering.
It may then be necessary for the sending node to resend lost or distorted data units to the receiving node. In order to perform a resending, the sending node in some way has to be informed which data, if any, to resend to the receiving node.
One mechanism that may be used in order for the sending node to know if data has to be resent is to poll the receiving node to send a status report back to the sending node.
The RLC protocol applied in an evolved UTRAN (E-UTRAN), also denoted Long Term Evolution (LTE), has been defined in the document 3GPP TS 36.322 “Evolved Universal Terrestrial Radio Access (E-UTRA), Radio Link Control (RLC) protocol specification Release 8” issued by the 3rd Generation Partnership Project (3GPP). The RLC protocol includes a polling procedure that transmits polls according to a number of criteria. When a poll is triggered the RLC transmitter will set a poll bit in the RLC header, the poll bit serving as a request for a peer entity to send an RLC status report. Currently agreed criteria for setting the poll bit are:
Firstly, transmission of last Protocol Data Unit (PDU) in a buffer, i.e. a poll is sent when the last PDU available for transmission or retransmission is transmitted.
Secondly, the expiry of a poll retransmission timer, i.e. a timer is started when a PDU containing the poll is sent and the PDU is retransmitted if the PDU with the poll bit is not acknowledged when the timer expires.
Such criteria for setting poll bits may work well for bursty traffic, where the poll is sent for the last PDU in each burst. For continuous transmission however, additional triggers may has to be considered. A properly designed polling procedure can be used to limit the number of outstanding, i.e. transmitted but not acknowledged, PDUs, or bytes, and to avoid stalling situations. Two mechanisms, counter-based and window-based, have been identified to avoid protocol stalling. Protocol stalling is an expression signifying that no more new data can be transmitted. Further, the polling mechanism may operate either on transmitted RLC PDUs or on transmitted bytes.
A counter-based mechanism counts the amount of transmitted PDUs, or bytes, and sets the poll bit when a configured number of PDUs, or bytes, have been transmitted.
A window-based mechanism is similar but transmits the poll only when the amount of outstanding data exceeds a certain number of PDUs, or bytes. A window-based mechanism may need additional logic to transmit the poll regularly as long as the amount of outstanding data exceeds the threshold.
However, none of the existing mechanisms does take into account that stalling sometimes may occur due to sequence number limitations and sometimes due to memory limitations. In particular, the buffer memory of a user equipment such as e.g. a mobile phone may be limited.
The user access quality and overall capacity in a wireless communication network environment is affected by data loss and protocol stalling, but also by unnecessary polls and resending of data.